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GSI-UPGRADE-ACC-05 GSI SCIENTIFIC REPORT 2009
Measurement of the Longitudinal Phase Space at UNILAC
To deliver the required currents for FAIR an important
goal is to improve the matching into the Alvarez section of
the UNILAC. Besides simulated phase space data, crucial
information about the beam parameters is provided by diagnostic
devices. While there are several standard methods
to access the four-dimensional transverse phase space information,
the longitudinal degree of freedom is as important
but an unequally more demanding measurement task.
The current approach is based on the time-of-flight
(TOF) measurement of single particles between two detectors
(see Fig. 1), and constructing the longitudinal phase
space via 2-dim histogramming using the arrival times at
both detectors and UNILAC rf reference [1]. The energy
information is extracted from the TOF between a MCP and
a diamond detector, whereas the arrival time at the diamond
with respect to the rf serves as a direct measure of the relative
phase location between particles. Imposed by the limited
distance (800 mm) between these detectors, a time resolution
of only several tens of picoseconds is mandatory to
resolve an rms energy spread of about 1% at 1.4 MeV/u.
The TDC and discriminators are capable to provide a time
resolution of about 50 ps rms. Additionally, signals from
the diamond and MCP suffer from a broad pulse height
spectrum and varying pulse shapes that significantly degrade
time resolution.
While the vertical projection of the longitudinal phase
space distribution yields trustworthy bunch structure information,
the limited energy resolution affects the measured
size of the phase space, which results in an enlarged
emittance ε and decreased covariance. Therefore
the related correlation � �as
described by the Twiss param-
∆E
eter α = −cov 〈E〉 , t /ε is smaller than expected from
simulations and a plausible machine operation mode. To
3
Coulomb
scattering at
thin Ta target Collimator
Gating Discriminators Amps
Anode
50
T. Milosic 1 , P. Forck 1 , and D. Liakin 1,2
1 GSI, Darmstadt, Germany; 2 ITEP, Moscow, Russia
MCP
2
e -
Diamond
detector
poly cryst.
S 1
Thin Al Al foil
Figure 1: Single particles are detected indirectly at the
MCP module (2) and directly at the diamond detector (1)
whereas the UNILAC rf serves as timing reference (3).
150
Relative Energy Deviation
Relative Energy Deviation
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
a)
Original Data (60k Events)
-6 -4 -2 0 2 4 6
Arrival time at Diamond detector [ns]
-6 -4 -2 0 2 4 6
Arrival time at Diamond detector [ns]
[Twiss Parameter]
α
Correlation
Relative Energy Deviation
2.5
2
1.5
1
0.5
0.08
0.06
0.04
0.02
0
-0.02
-0.08
Original Data
Vertical Fit
Deconvolved
10 20 30 40 50 60 70 80 90
Cutoff [% Integral Intensity]
Vertical Gauss Fit
-0.04
-0.06 Analytic Deconvolution
d)
b) c)
-6 -4 -2 0 2 4 6
Arrival time at Diamond detector [ns]
Figure 2: a) Plain data from DAQ. b) Reconstructed phase
space using Gaussian fits to energy degree of freedom.
c) Analytic deconvolution with σi,dec =
�
σ 2 i,fit − σ2 res
for each vertical slice i (see text). d) Correlation α for
original, fitted and deconvolved phase space vs. integral
cutoff levels.
improve the systematic energy broadening, deconvolution
methods were investigated in order to disentangle the error
contribution from the original phase space data. In Fig. 2
an example of the original and deconvoluted data is shown
from a high current Argon beam (6.5 mA) recorded during
the HIPPI campaign at GSI [2]. Statistical noise, off-center
vagabonding particles and the intrinsic low count rates are
severe issues concerning a deconvolution approach. Furthermore,
no pulse response is available from experimental
data due to the lack of a monochromatic beam. Thus
the pulse response is deduced from a consideration of error
contribution based on a Gaussian model denoted by σres.
This permits us to perform an analytic deconvolution of the
energy information when vertical slices are also fitted with
an Gaussian parameterization as depicted in Fig. 2. A consistent
deconvolution of the energy distribution is achieved
even for noisy data. Moreover, the correlation α changes
significantly and approaches the expected value.
As an alternative to the TOF measurement preliminary
investigations of the calorimetric properties of a monocrystalline
diamond detector have been performed using an
Am-243 source. Preliminary analysis shows an energy resolution
of about 1% for α particles at several MeV. The
detector has been installed and will be tested soon.
References
[1] T. Milosic, P. Forck, D. Liakin, “Longitudinal Emittance
Measurement using Particle Detectors”, DIPAC’09, Basel,
May 2009, p. 330.
[2] High Intensity Pulsed Proton Injectors (HIPPI), Joint Research
Activity (JRA3) in the framework of CARE.